1. Field of the Invention
The present invention relates to a method for producing a positive active material of a lithium secondary battery using an organic electrolytic solution, a polymer electrolyte or an inorganic solid electrolyte.
2. Description of the Related Art
At present, a so-called lithium ion secondary battery using a carbon material as a negative electrode, lithium cobaltate (LiCoO.sub.2) which is a lithium intercalation compound having a layered structure as a positive electrode and an organic electrolytic solution as an electrolyte has been put into practice. This battery is used in various kinds of portable electronic appliances because it has very high energy density. Lithium cobaltate as a positive active material has several advantages such as high operating voltage 4V vs. Li/Li.sup.+ ; high practical specific capacity of about 150 mAh/g; and good cyclic reversibility. However, cobalt is expensive because of the limited reserve in view of natural resources. Therefore, lithium nickelate (LiNiO.sub.2) could be an alternative replacement to lithium cobaltate.
Similar to lithium cobaltate, lithium nickelate has a hexagonal layered structure belonging to a space group R3m. In lithium nickelate, the potential is about 4V vs. Li/Li.sup.+ and the practical specific capacity is about 200 mAh/g which is higher than that of lithium cobaltate. When this material is used as a positive electrode, however, multi-stage processes take place with the cell potential showing 4 plateaus indicating a four-phase reaction due to the occurrences of several structural transitions during charging and discharging. As a result, the electrochemical performance of the battery degraded very quickly upon cycling, for example, as reported in Solid State Ionics, 44, 87 (1990). Accordingly, although the specific capacity was of a very large value about 200 mAh/g initially, it decreases greatly upon repeated charging/discharging. In charge and discharging curves, the structural transition takes place with the presence of 4 plateaus in the potential curve.
In order to suppress the structural transition upon charging and discharging, it is effective to replace a part of nickel with another element. For example, replacement of nickel with cobalt has been reported in Chem. Express, 6, 161 (1991). In this case, a high-temperature solid phase sintering method was used in which: aqueous solutions of Ni(NO.sub.3), Co(NO.sub.3).sub.2 and LiOH are mixed; and the resulting mixture is dried at 90.degree. C. in advance, and then sintered at 800.degree. C. in air. Further, replacement of a part of nickel with manganese has been reported in Solid State Ionics, 57, 311 (1992) and replacement of a part of nickel with an alkali-earth metal (at the rate in a range of 0.05 to 0.10) such as magnesium, calcium, strontium, barium, etc. has been reported in the 36th Battery Discussion Meeting Lecture Summary, 9, 17, (195). In any case, the fading of capacity with the charging/discharging cycle has been limited compared with the case of pure LiNiO.sub.2. There is, however, a problem that the initial specific capacity is relatively low.
Generally, when a high-temperature solid phase sintering method is employed at about 750.degree. C., the specific capacity density becomes low because vaporization of lithium occurs so that stoichiometrical active material cannot be obtained easily. In the solid sintering method, the sintering temperature should be lowered so as to avoid the vaporization of lithium at a high temperature. In this case, however, lithium nickelate cannot be obtained successfully. For improvement of this point, there is an effective countermeasure in which not respective salts of nickel and cobalt but a complex oxyhydroxide of nickel and cobalt is used as a starting material, for example, as described in the 36th Battery Discussion Meeting Lecture Summary, P.65 (1995). That is, although both nickel salt and cobalt salt are ordinarily divalent, both Ni and Co must be oxidized to trivalent state in order to synthesize cobalt-containing lithium nickelate represented by the composition LiNi.sub.1-x Co.sub.x O.sub.2. The oxidation of each of Ni and Co from divalent to trivalent state cannot be achieved at a low temperature. On the contrary, if Ni and Co are provided in the form of oxyhydroxide (Ni.sub.1-x Co.sub.x OOH), LiNi.sub.1-x Co.sub.x O.sub.2 is generated even at a low temperature of 400 to 500.degree. C. when oxyhydroxide (Ni.sub.1-x Co.sub.x OOH) is made to react with a lithium compound because both Ni and Co are trivalent.
In the low-temperature solid phase sintering method, very high specific capacity of 190 mAh/g can be obtained when nitrate is used as the lithium compound whereas a specific capacity not higher than about 160 mAh/g can be obtained when hydroxide is used. This is because lithium nitrate is melted at a temperature of about 253.degree. C. and then reacts with oxyhydroxide easily whereas no melting takes place when lithium hydroxide is used. As described above, lithium nitrate is an excellent starting material in terms of improvement of the specific capacity but has a disadvantage in that harmful NO.sub.x gas is generated in the solid sintering process. Further, high specific capacity is obtained when a mixture gas of argon (80%) and oxygen (20%) is used as an atmospheric gas in the sintering process. However, when air, used as the sintering atmosphere there arises a problem that the specific capacity is reduced because lithium carbonate is generated as impurities due to the presence of carbon dioxide.
On the other hand, as a method of synthesizing a lithium secondary battery active material, there is used a so-called hydrothermal method in which a reaction can be made to progress, for example, at a low temperature not higher than 250.degree. C. and at a high pressure. The hydrothermal method is, however, heretofore applied only to synthesis of LiFeO.sub.2 (Solid State Ionics, 79, 1369 (1995)) and synthesis of LiMnO.sub.2 (Proc. First Scientific Workshop for Electrochem. Materials, p.75 (1996)), for example, by using an iron compound and lithium hydroxide as starting materials.
As described above, the method of synthesizing lithium nickelate LiNi.sub.1-x Me.sub.x O.sub.2 partially replaced by other elements cannot simultaneously satisfy: (A) high specific capacity; (B) prevention of generation of NO.sub.x gas at the time of synthesizing; and (C) use of air as an atmosphere at the time of synthesizing.